Here is information available online regarding foster care and kinship care in Idaho, and while I do not have experience with ID, it appears to be pretty similar to the systems in most other states.
Kinship and Caregiving | Idaho Department of Health and Welfare
Child Protection and Foster Care | Idaho Department of Health and Welfare
Frequently Asked Questions | Idaho Department of Health and Welfare
In most situations, a biological parent has to be pretty significantly incapable of parenting a child for that child to be removed from their custody. A parent can also voluntarily give up custody to another party, but for it to be a true foster care placement, then that state's CPS or equivalent has to be involved. Anyone can become a foster parent, if they meet the criteria for certification and receive the prescribed training for that state. Certification involves, at the very least, a criminal background check and a child welfare background check, as well as passing a physical and having a home that meets minimum standards for that state. Although my background is in NY's child welfare system, I am pretty confident that these basic elements of licensing are uniform across the country because there is federal funding involved.
CS may have been appropriate when the children were first placed with her, and things may have only recently gotten bad in her home. She also may have been able to hide a lot of things -- covid has had a fierce impact in our ability to be in people's homes and lives, and we have to rely more heavily on the other entities in a person's life to help supervise what is going on, like teachers and medical providers and the community itself.
Caseworkers aren't omniscient beings who can see all things, yet choose to sit back with their feet up and ignore reality. People don't choose that line of work because it's easy or financially rewarding. Are there stories of horrible hardened Caseworkers in their bad sweaters and half-glasses sitting around drinking coffee and smoking cigarettes and ignoring their duties? Yes, but they are actually becoming fewer and fewer with the efforts states have been putting into improving performance and finding innovative ways to successfully work within today's challenges.
It isn't known that CS was actually licensed as a foster parent. She may have obtained custody of those children without CPS involvement and "fostering" is just a vernacular term that is now being used in the media as an explanation for this particular living arrangement. Just because a person has custody of a child who is not their own doesn't merit CPS involvement, there has to be a reason for them to insert themselves into someone's life.
With all of that said, if this is a situation of a Caseworker being negligent, I fully support that person being crucified. These kids deserved so much more from this world.